Microcavity plasma devices produce a nonequilibrium, low temperature plasma within, and essentially confined to, a cavity having a characteristic dimension d below approximately 500 μm, and preferably substantially smaller, down to about 10 μm (at present). Such microplasma devices provide properties that differ substantially from those of conventional, macroscopic plasma sources. Because of their small physical dimensions, microplasmas normally operate at gas (or vapor) pressures considerably higher than those accessible to macroscopic devices. For example, microplasma devices with a cylindrical microcavity having a diameter of 200-300 μm (or less) are capable of operation at rare gas (as well as N2 and other gases tested to date) pressures up to and beyond one atmosphere.
Such high pressure operation is advantageous. An example advantage is that, at these higher pressures, plasma chemistry favors the formation of several families of electronically-excited molecules, including the rare gas dimers (Xe2, Kr2, Ar2, . . . ) and the rare gas-halides (such as XeCl, ArF, and Kr2F) that are known to be efficient emitters of ultraviolet (UV), vacuum ultraviolet (VUV), and visible radiation. This characteristic, in combination with the ability of microplasma devices to operate in a wide range of gases or vapors (and combinations thereof), offers emission wavelengths extending over a broad spectral range. Furthermore, operation of the plasma in the vicinity of atmospheric pressure minimizes the pressure differential across the packaging material when a microplasma device or array is sealed. Operation at atmospheric pressure also allows for arrays of microplasmas to serve as microchemical reactors not requiring the use of vacuum pumps or associated hardware.
Research by the present inventors and colleagues at the University of Illinois has resulted in new microcavity and microchannel plasma device structures as well as applications. Recent work has resulted in microcavity and microchannel plasma devices that are easily and inexpensively formed in metal/metal oxide (e.g., Al/Al2O3) structures by simple anodization processes. Large-scale manufacturing of microplasma device arrays benefits from structures and fabrication methods that reduce cost and increase reliability. Of particular interest in this regard are the electrical interconnections between devices in a large array as well as the reproducible formation of electrodes having a precisely-controlled geometry.
The metal-metal oxide microplasma device arrays developed prior to the present invention have been formed by joining at least two sheets. Each separate sheet, e.g. a foil or screen, contains one of the two required driving electrodes for generating plasmas. These prior arrays work very well, but having two sheets typically requires alignment and bonding of the two pieces, and especially so if addressable arrays are to be formed. Precision alignment becomes challenging and potentially costly when the alignment error must be a small fraction of the microcavity cross-sectional dimension (typically 10-200 μm). Also, the bonding of separate electrode sheets can reduce the array lifetime because bonding increases the probability for electrical breakdown along the surface of one of the electrode.